tests/hexloader-test: Don't pass -nographic to the QEMU under test

The hexloader test invokes QEMU with the -nographic argument. This
is unnecessary, because the qtest_initf() function will pass it
-display none, which suffices to disable the graphical window.
It also means that the QEMU process will make the stdin/stdout
O_NONBLOCK. Since O_NONBLOCK is not per-file descriptor but per
"file description", this non-blocking behaviour is then shared
with any other process that's using the stdin/stdout of the
'make check' run, including make itself. This can result in make
falling over with "make: write error: stdout" because it got
an unexpected EINTR trying to write output messages to the terminal.
This is particularly noticable if running 'make check' in a loop with
  while make check; do true; done
(It does not affect single make check runs so much because the
shell will remove the O_NONBLOCK status before it reads the
terminal for interactive input.)

Remove the unwanted -nographic argument.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>